Thermally responsive indicators are useful in a variety of fields for providing a visual indication of the attainment of a desired temperature. For example, U.S. Pat. No. 4,083,364 discloses a thermal indicator mounted through the skull of an animal to detect the presence of an elevated body temperature. German Patent No. 3,229,020 discloses a thermally responsive indicator that is designed to be mounted on an electrical conductor. The device provides a visual indication of the attainment of an elevated temperature in the conductor. U.S. Pat. No. 4,818,119 discloses a railroad wheel bearing bolt with an axially extending bore into which a heat sensor and indicator are inserted. The indicator is exposed when a desired temperature is attained Another example can be found in U.S. Pat. No. 4,289,088, which discloses a sterility indicating device for use in a steam autoclave.
Thermally responsive indicating devices are especially useful in preparing food products, particularly meat and fowl. Such devices are useful for indicating the elevated temperature of the interior of the food product, rather than the temperature of the exterior thereof. By indicating the attainment of a desired internal temperature of the food product, such devices can signal when the food product is organoleptically acceptable. Furthermore, these devices must be sufficiently accurate to prevent undercooking or overcooking, which significantly diminishes the palatability of the food, and may even render the food dangerous to eat, as in the case of undercooked pork.
Several such devices for use in cooking food are known. Examples of early indicating devices are described in U.S. Pat. Nos. 945,978 and 1,509,110, each of which allows a plunger biased by a spring to be released into an extended position upon attaining a predetermined temperature. Retaining means, which is usually a fusible material, holds the plunger in a retracted position until the fusible material yields, at which time a spring urges the plunger into an extended position. When in the extended position, the plunger provides a visual indication to the user that the food is "done." To further enhance the visibility of the plunger when it is in the extended position, a cap may be attached to the end thereof.
The retaining material has typically comprised metal alloys, as in U.S. Pat. Nos. 3,140,611, 3,682,130, 3,693,579 and 3,713,416. The '611 patent, for example, refers to an alloy "consisting of bismuth--52%, lead--40%, cadmium--8%, to which is added two parts of Wood's alloy." Indicators that use such alloys include the "Vue-Temp" brand and "Pop-Rite" brand pop-up timers by the Volk Development Company of Turlock, Calif. 95381. While devices employing such alloy retaining materials have proven useful, they have certain drawbacks. For example, such alloys are typically prepared from toxic metallic substances such as bismuth, lead, cadmium, tin and other materials, as noted above. Furthermore, metal alloys are high density materials. Thus, relatively heavy amounts of alloy must be used in order to provide a given volume of retaining material, thereby adding to the expense of the device.
Organic compounds, such as waxes, have been suggested as replacements for metal alloys but have not proven to be satisfactory. Waxes typically yield over a relatively wide temperature range and if composed of a mixture of different materials, can have multiple melting temperatures, thereby resulting in an early or a delayed signal. This, as mentioned above, is undesirable because the food item could either be undercooked or overcooked, respectively.
A known device which attempts to overcome the problems exhibited by alloy and wax retaining materials is described in U.S. Pat. No. 4,170,956. The '956 patent discloses a nitrogen-containing organic retaining material. Devices using these nitrogen-containing materials have been sold in the United States under the name "Dun-Rite" brand pop-up timers by Minnesota Mining and Manufacturing Company of St. Paul, Minn. The nitrogen-based materials used in these devices do not exhibit the toxicity problems of their alloy predecessors and exhibit better release characteristics than waxes. However, while having their own utility, the nitrogen-based materials disclosed in the '956 patent also have certain drawbacks.
For example, the nitrogen-based materials of the '956 patent are very pure when they are prepared. Melting temperature (and therefore yield temperature) tends to increase with increased purity, and the materials of the '956 patent melt at approximately 84.9.degree. C. when they are tested in their purest state. A turkey, for example, is considered "done" when the interior temperature reaches approximately 82.2.degree. C., and the retaining material must therefore be mixed with a foreign substance to reduce its purity. The foreign substances are often an excess amount of one of the constituent elements of the nitrogen-based material, which adds unnecessary expense. Salts, which are added to the material to clean it, tend to solidify to form gel globules in the material which necessitates that the material be strained in order to obtain functionally acceptable material. The straining process often must be conducted repeatedly, which increases the time and cost of production. The available yield is reduced, and consequently, a greater amount of material must be manufactured and processed to obtain a given amount of retaining material.